The past few years have seen an explosive growth in the number of mobile devices such as cellphones, PDAs, and laptop computers. These devices can use a variety of wireless access technologies. These range from wide-area technologies such as GPRS, EDGE, CDMA 1xRTT, EV-DO, and satellite access, to local-area technologies such as 802.11a/b/g and short-range technologies such as Bluetooth, Zigbee, etc. However, any wireless access technology must make a difficult tradeoff between the coverage of an access point and the capacity available to a user in that access point's coverage area. To offer wireless access in a given geographical area, wide-area wireless access technologies require fewer access points but offer inherently lower per-user capacity. Short-range access networks can offer large per-user capacity, but the capital cost to offer coverage in large geographical areas can be prohibitive. Consequently, no single wireless access technology can be expected to provide ubiquitous, high-bandwidth coverage. For example, high-speed 802.11 a/b/g access coverage is typically confined to WLANs inside buildings and public hot-spots. In contrast, lower-speed WWAN technologies such as CDMA 1xRTT and GPRS provide far wider coverage, although even such technologies cannot be expected to be available everywhere and coverage can be decidedly spotty inside enclosed areas. Although city-wide 802.11 mesh network deployment tries to provide ubiquitous wireless broadband access, the success is limited because of external interference in the 2.4 GHz band, and a reduction in capacity when the multi-hop count is large. Note that, besides this coverage-capacity tradeoff, managed wireless technologies impose limits on the number of simultaneous users in a given geographic area. This may prevent a user from using a network even when it is available.
Fortunately, current and future mobile devices are likely to come equipped with multiple wireless interfaces that can be used either singly or in parallel.